In recent years, in addition to manual driving in which a vehicle travels based on user's driving maneuvers, a new drive mode has been proposed regarding autonomous driving assistance systems that assist the user in driving a vehicle by performing a part or all of user's driving maneuvers by the vehicle. For example, the autonomous driving assistance system autonomously performs vehicle control such as control of steering, a driving source, and a brake so as to keep the vehicle approximately centered in the same lane while maintaining a preset vehicle speed and a certain distance to a vehicle ahead. The autonomous driving assistance system is advantageous in that it eases the driving burden on the user. However, in order to allow the vehicle to appropriately travel with autonomous driving assistance, it is important to more quickly and accurately specify a route to be taken by the vehicle.
For example, as shown in FIG. 16, in the case where a straight road 102 and a curved road 103 are connected at a branch point ahead of a vehicle traveling on a road 101, control for autonomous driving assistance varies depending on which way the vehicle is going to go at the branch point. That is, in the case where the vehicle is going to take the curved road 103, the vehicle needs to be decelerated to a speed according to the radius of curvature of the curve by the time the vehicle enters the curve. In the case where the vehicle is going to take the straight road 102, such deceleration is not needed, and it is desirable that the vehicle travel at as constant a speed as possible so as not to impede traffic flow. Accordingly, appropriate autonomous driving assistance cannot be executed unless a route to be taken by the vehicle can be specified. For example, International Publication No. 2011/158347 (WO 2011/158347) proposes a technique in which, in the case where a destination has been set, autonomous driving assistance is executed by using a route to the destination as the course of the vehicle, and in the case where no destination has been set, autonomous driving assistance is executed by using a route along the road as the course of the vehicle.